1 electron = 1 photon?

Started Apr 12, 2013 | Discussions thread
alanr0 Senior Member • Posts: 2,073
APD, EMCCD and Compton scattering

Jack Hogan wrote:

alanr0 wrote:

The response is shown in A/W.  For each electron released, the charge is constant, but the photon energy is proportional to the photon frequency, and inversely proportional to the wavelength.

Efficiency in A/W = (electron charge x wavelength)/(planck's const x velocity of light)

At 100% quantum efficiency (1 electron per photon), you get 0.645 A/W at 800 nm, but only 0.323 A/W at 400 nm.  Your graph shows the quantum efficiency falling from around 84% at 600-800 nm, to around 53% at 400 nm.

In other words, less than 100% QE, with peak performance at 600-800 nm.


Very helpful as always, Alan, I assume this holds on average.   Can there ever be a case where a single particularly energetic photon results in two electrons being produced by the photodiode?

If the photon has sufficient energy, more than one electron can be released.  PhotonTrapper has identified possible mechanisms, such as double electron photoemission.  I don't believe this or other mechanisms make a significant contribution when conventional silicon photodiodes exposed to visible light.

At much higher (X- and gamma-ray) photon energies, Compton scattering can release multiple electrons as a photon propagates through the material.  At each scattering event, an electron-hole pair is created, and the photon loses some energy.  Consult Storm & Israel's tables for the dependence on atomic number and photon energy if you don't have access to code like this.

A different mechanism operates in an avalanche photodiode (APD).  Arguably there are two processes operating sequentially here.  An electron-hole pair is first generated by photo-absorption.  APDs operate at much higher bias voltages than conventional photodiodes, and the carriers are accelerated by the bias field, releasing further electron-hole pairs by avalanche multiplication.

Impact ionisation is also exploited by electron-multiplying EMCCD detectors, to produce multiple electrons from a single absorbed photon.


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Alan Robinson

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